This application is related to the copending application entitled Synchronous Power Transfer Ser. No. 37,680 filed on May 10, 1979 now U.S. Pat. No. 4,256,972, in the names of Michael A. Wyatt and Robert W. Beckwith.
Changing electrical power system needs in the last decade have placed a more stringent demand for the rapid transfer of power to a motor bus having an inductive residual voltage thereon. For example, note FIG. 1 which shows a sketch of a typical power plant system 9. Under normal operating conditions circuit breaker 11 is closed, circuit breaker 12 is open, and accordingly power is supplied to the motor bus 15 through the main transformer 16. As is known, the auxiliary source or system 17 functions as a back-up or stand-by system. Under certain conditions, such as, for example, when a fault occurs in the main power source or system 18, such as between the generator 21 and the mainpower transformer 19, breaker 12 will open. In such event, it is desirable to electrically connect the motor bus 15 through the auxiliary transformer 20 to the auxiliary system 17 by closing breaker 14.
A fast transfer method is required to quickly establish the closing of breaker 14 to the auxiliary system 17. For example, in a nuclear plant, this auxiliary system must be connected or established within less than one second; otherwise, the plant will automatically shut down. In a coal fired plant, the time may even be shorter since slowing down of the forced draft fan can cause a boiler explosion due to the inertia of the air coming into the boiler.
Various methods are currently used for providing such transfer of power from the main system to the auxiliary system. One method has been to utilize two circuit breakers, one breaker coupling from the motor bus to the main source of power and the second breaker coupling from the motor bus to the auxiliary system. Closing of the second breaker is initiated, and the tripping or opening of the first breaker is accompanied by the same electrical contact. This requires the closing time of the second breaker to always be slower than the tripping time of the first breaker. If for some reason the foregoing does not occur in the exact time sequences stated, fault current can flow momentarily through the station service transformers resulting in the dramatic failure of the transformers.
A second method has been to initiate closing of a second breaker with a contact on the first breaker. However, in this method the transfer time may often be so long as to produce a worst case phase angle between the induced motor voltage and the new source. This can produce damaging stress on the motor shafts.
Another prior art method for closing the breaker 14 is a so-called residual voltage transfer. In this latter method, after the breaker 12 has opened, the voltage on the motor bus 15 is allowed to decay to a value of about 25% of the motor rated voltage, and then breaker 14 is closed. In starting up the plant, motors are started one, or a few, at a time. The auxiliary system 17 will generally be incapable of starting all the motors at once or even reaccelerating them after a short power interruption. The method of waiting for a 25% voltage level therefore requires selectively shutting down some non-essential motors in order to hopefully be able to reaccelerate critical motors. It is most difficult to assure the success of this method under the worst case combination of all the variables existing at the instant a transfer is initiated.
When breaker 12 is closed at zero degrees, it can be shown that all the inrush current goes to smoothly reaccelerating the motors and none is converted to potentially damaging shaft torque transients.
As shown in FIG. 2, the motor bus 15 residual voltage decay can be approximated as a straight line down to at least 25% value of motor rated voltage. The point t.sub.1 and t.sub.2 represent the time intervals for two different motor loads, of different characteristics, connected to the buses. It is important to note that the voltage decay rate is a function of the motor load characteristics, and accordingly, it becomes impractical to predict what the voltage decay rate will be at any time since the load combination on the motor bus is not known.